A wireless LAN (Local Area Network) utilized when a wireless terminal device performs wireless communication now contains access points connected to a wired network [for example, see IEEE 802.11 (1999 Edition)]. In a wireless LAN, there is an infrastructure mode in which an access point arbitrates the access timing of a plurality of wireless terminal devices in a wireless network, and an ad-hoc mode in which access timing is decided between wireless terminal devices.
The infrastructure mode is disadvantageous in that throughput declines in comparison with the ad-hoc mode because of the intervention of the access point. However, access to a wireless LAN or to the Internet can be achieved. With the ad-hoc mode, on the other hand, throughput rises because wireless terminal devices communicate directly. A disadvantage, however, is that a wired LAN and the Internet cannot be accessed. In order to exploit the advantages of both modes, a communication scheme that switches between the infrastructure mode and the ad-hoc mode has been proposed. For example, see the specification of Japanese Patent Application Laid-Open No. 2003-249939 (Prior Art 1).
Further, schemes compliant with IEEE 802.11e/D8.0 (February 2004) are presently being studied in order to implement real-time data communication by the wireless LAN mentioned above. Proposed among these schemes for the purpose of exploiting the advantages of the infrastructure mode and ad-hoc mode is a Direct Link Protocol mode (referred to as the “DLP mode” below) in which wireless terminal devices communicate directly even in the infrastructure mode.
However, in communication in which the infrastructure mode is switched to the ad-hoc mode and communication in which use is made of the DLP mode, a case is conceivable in which despite the fact that communication is possible via an access point, radio waves may not reach from one directly communicating wireless terminal device to another owing to the conditions at the locations where the wireless terminal devices are placed.
Accordingly, the following methods have been proposed as methods of verifying whether direct communication is possible or not:
(1) a method of sending a communicating party a request for performing direct communication via an access point and determining whether or not direct communication is possible based upon whether or not it is possible to confirm whether or not a reply signal can be received [e.g., see the specification of Japanese Patent Application Laid-Open No. 2003-348103 (Prior Art 2)]; and
(2) a method of receiving beforehand all radio waves issued by other wireless terminal devices and creating a list of wireless terminal devices from which radio waves could be received, thereby determining whether or not a wireless terminal device is capable of direct communication [e.g., see the specification of Japanese Patent Application Laid-Open No. 2004-072565 (Prior Art 3)].
In the examples of the prior art set forth above, whether or not radio waves will arrive, i.e., whether or not communication is possible, can be determined. However, a case is conceivable in which even though radio waves actually arrive, the wireless link rate must be lowered owing to a poor environment at the time of radio-wave propagation. In such case it is conceivable that throughput, rather than rising, will fall in comparison with communication via an access point. Further, in the case of Prior Art 2 or Prior Art 3, it is necessary to receive all receivable wireless packets over a certain period of time in order to acquire the type of wireless packet or address of the wireless terminal device that is the source of transmission. As a consequence, this processing constitutes a very heavy load. In addition, owing to the heavy load, there is the possibility that no all information will be acquired.
Furthermore, with the prior art described above, whether or not a wireless terminal device capable of direct communication exists can be ascertained. However, unless communication is actually carried out, it is not possible to determine whether an improvement in throughput owing to direct communication will be obtained as compared with a case where communication is performed via an access point.